Since a resin optical material is lighter and less fragile than an optical material made of an inorganic material and tintable, it has been recently rapidly accepted for various optical materials, such as a spectacle lens and a camera lens.
While, a resin for the optical material with higher performance has been demanded. More particularly, there has been a need for improvements toward a higher refractive index, a higher Abbe's number, a lower density and a higher heat resistance. Responding to such a need, various resins for the optical material have been developed and put into use.
Among others, various polyurethane resins have been proposed actively. The present inventors have also made many proposals concerning optical materials, such as a plastic lens, made of a polyurethane resin.
As one of the most representative resins of the polyurethane resin, a resin obtained by reacting a polythiol compound and a polyiso(thio)cyanate compound may be exemplified. The resin is transparent and colorless, has a high refractive index and a low dispersion property, and is excellent in properties, such as an impact strength, tintability and processability. Consequently, it is one of the best resins for the optical material such as a plastic lens. Among others, the resin transparency is a crucial property for a lens.